The present invention relates to the resonance magnetic arts. It finds particular application in conjunction with diagnostic imaging at surgical sites and will be described with particular reference thereto. However, it is to be appreciated that the invention will also find application in other magnetic imaging, spectroscopy, and therapy applications.
Early magnetic resonance imaging systems were based on solenoid magnets, A series of annular magnets were placed around the bore through which a magnetic field was generated longitudinally. A patient was selectively moved axially along a horizontal axis of the bore to be positioned for imaging. Magnetic resonance imaging systems with solenoid magnets tended to be claustrophobic. Moreover, access to the subject for surgical procedures, minimally invasive procedures, physiological tests, equipment access, and the like was limited and awkward.
To provide for patient access and reduce the claustrophobic effects, open or vertical field magnets have been devised. Open magnets typically include an upper pole, a lower pole, and a ferrous flux path connecting them. The ferrous flux path is typically in the form of a C, an H, a four poster arrangement, ferrous plates in ceilings and walls, or the like. Although less energy efficient, the magnetic flux can return through the air or through other constructions with less magnetic susceptibility than a ferrous return path.
In order to improve the homogeneity of magnetic flux through the gap between the pole pieces, various constructions have been provided at the poles. Typically, each pole includes a large ferrous pole piece which may be shaped or contoured to improve magnetic flux uniformity. Typically, a heavy ferrous ring, known as a Rose ring, was positioned around the circumference of the pole piece, often extending beyond the pole piece towards the patient to drive the magnetic flux towards the central axis of the patient receiving gap. The magnet is often positioned offset toward the flux return path form the Rose ring.
Typically, a super-conducting magnet has been provided at each pole. In the presence of a magnetic field, the pole pieces, Rose ring, and super-conducting magnets attracted towards each other with a significant force. It has been found that when this pole assembly is spaced from the ferrous return path, there is an opposite attraction toward the ferrous return path. Balancing or zeroing the forces on the pole assembly permits lighter weight constructions to be used to support the pole assemblies. However, the present inventors have found that when magnets ramp up and ramp down, the ramping process is not linear. Rather, during a typical ramp up period, the pole assemblies will at times be attracted toward each other and at other times be attracted toward the ferrous flux return path. This oscillation between opposite forces tends to fatigue and stress the support assembly.
Another disadvantage of placing the coil at the axially position of balanced force is that the axial force changes with changes in the field. When the coil is shifted from the balanced force position, the force to increase the shift grows rapidly. This creates a tendency for a small vibrational shift to cascade into catastrophic shift forces. To control and avoid such a runaway situation, the coil supports are constructed with very high stiffness.
In the prior art coils, the Rose ring was typically shifted axially relative to the coil. With the Rose ring positioned closer to the subject than the coil, the Rose ring tends to saturate with increasing field. This change in Rose ring saturation, again causes changes in the relative force balance.
This application provides a new and improved magnetic resonance imaging system which overcomes the above referenced problems and others.
In accordance with one aspect of the present invention, a magnetic resonance system includes a pair of pole assemblies disposed on opposite sides of an examination region. At least one of the pole assemblies includes an annular magnet and an annular Rose ring assembly disposed concentrically within the annular magnet. The annular magnet is mounted such that it is axially centered relative to the Rose ring assembly.
In accordance with another aspect of the present invention, a magnetic resonance system includes a magnetic field source which generates a magnetic field axially through an examination region. A Rose ring is disposed adjacent the examination region and positioned such that the magnetic field extends axially through the Rose ring. The Rose ring includes at least two axially displaced Rose ring segments.
In accordance with another aspect of the present invention, a magnetic resonance method is provided. A magnetic field is induced axially through an examination region. The homogeneity of the magnetic field is improved with an annular Rose ring having a plurality of axially separated Rose ring segments surrounding the magnetic field.
One advantage of the present invention is that it facilitates the design of open magnets with stronger magnetic fields.
Another advantage of the present invention is that it improves magnetic field homogeneity.
Yet another advantage of the present invention is that it improves structural stability.
Yet another advantage resides in the low forces on driver coils.
Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.